1. Field of the Invention
The present invention is generally related to railway vehicles having a variable trim body, i.e. adapted to be rotated about its longitudinal axis when the vehicle is running along curved paths of the railway track, towards the inside of the curve. These rotations or rolls of the body enable, particularly in the case of high-speed railway vehicles, to appreciably enhance comfort for the passengers, due to the fact that the transverse acceleration felt inside the body while the vehicle is running along a curve is relatively limited.
2. Description of the Prior Art
The body tilt variations are traditionally servo-operated by means of a body roll control system.
U.S. Pat. No. 3,844,225 assigned to Fiat Spa discloses such a control system for a railway vehicle including two bogies and resilient suspension means between the body and the bogies. This control system comprises two pairs of fluid pressure linear actuators substantially vertically interposed between the body and the opposite sides of the two bogies, a source of fluid under pressure, valve means for controlling communication between said actuator means and said source of fluid under pressure through respective fluid lines, a regulation electronic unit operatively associated to said valve means, and transducer means for detecting the non-compensated centrifugal accelerating acting on the vehicle body and for transmitting corresponding output signals to said regulation electronic unit to pilot said valve means, so as to perform, while the vehicle is running along a curve, rotations of the body about a longitudinal axis tending to compensate said centrifugal acceleration.
In conventional control system of the above-referenced type the fluid circuits of the actuator pairs associated to the two bogies of the vehicle are distinct and separate from each other, i.e. in practice independent relative to each other, two respective autonomous solenoid-valve assemblies being provided, which are piloted by the regulation electronic unit following processing of the output signals supplied by the transducer means.
When the vehicle is travelling along a straight path, the two solenoid-valve assemblies keep the actuator of the one and of the other pair isolated. Whenever the vehicle enters the parabolic entry transition section of a curve, the following problem is to be faced.
Upon encountering a track skew, i.e. of the slanting in vertical projection of the outside rail relative to the inside rail in correspondence of the entry transition section of the curve superelevation, the front bogie, with reference to the travel direction, rolls in the vertical plane and transmits to the body a rotation couple, through the corresponding secondary suspension. In such a condition, the suspension located in correspondence of the bogie side which is raised owing to the track skew is contracted, while the suspension located in correspondence of the opposite side tends to be unloaded. In the area of the front bogie a counteracting couple is thus produced, which generates an opposite reaction couple onto the rear bogie, through the corresponding secondary suspension. Therefore, the skew in the entry curve transition section normally produces an unbalanced load distribution over the secondary suspensions of the two bogies, and thus an uneven "unloading" effect of the respective axles.
Simultaneously, while the vehicle is entering the entry curve transition section, actuation of the body roll control system is started by means of the output signals generated by the transducer means, i.e. the actuators of the two bogies are operated. Since as pointed out the two fluid circuits of these actuators according to the prior art are independent from each other, owing to hydraulic or control differences or other kinds of variations or non-symmetries, the couples correspondingly applied by the actuators over the two bogies will normally be different. These different couples differently load the bogie axles, and consequently one axle (and thus the corresponding wheel-rail contacts) may be excessively unloaded. In practice, this situation corresponds to the self-generation by the vehicle itself of an additional skew which actually does not exist, i.e. of an "autogenous skew", which makes the above described situation, deriving from the real presence of a track skew in the entry curve transition section, remarkably worse.
This situation, which negatively affects the actuating condition of the control system also when the vehicle is running along a full curve as well as along the exit curve transition section, after all reduces travel intrinsic safety of the vehicle during operation of the body roll control system.